LMO tuning in the SB- series is by a small pinch wheel on the tuning shaft engaging a metal ring on the dial. The bushing for the tuning shaft can be moved up and down to change the amount of "mesh" and will often fix slipping dial problems. However, there are other things to keep in mind....

In some of these older Heathkit rigs the grease on the LMO tuning cap stiffens up such that turning the knob requires too much force. While you have the main tuning knob off, try tuning manually by using a small screwdriver through the 1/4" hole that is revealed under the knob. This allows you to turn the LMO shaft directly rather than via the friction drive.

The main tuning dial has a spiral groove on the back and a nylon follower that moves the 100's dial at the top. Often the main dial cracks causing the follower to hang up at certain spots or the follower is damaged or wears out causing a bind at certain spots on the dial. Look for cracks as the 15 and 90 positions of the main dial.

I've also seen cases where the main dial is rubbing on the cables, either the dial light cable or the big cable that runs underneath the chassis. This can cause binding and dial slippage as well (along with wear on the outer edge of the dial and the cable itself).

While you are in there poking around anyway, look to see if the clearance between the nylon follower for the 100's dial has the same clearance to the main tuning dial at both ends of its travel. If the LMO is not aligned square to the front panel, the clearance will be different at the in edge of the spiral vs. the outer edge and may result in binding.

Be careful about moving the tuning shaft and its pinch gear such that it puts excessive pressure on the metal drive ring on the main tuning dial. This can contribute to stress cracking in the already stress-crack prone main dial.

The "O-Rings" on the Preselector and Final Load are often in need of replacement but have nothing to do with the main tuning. The problem is that after a while the o-rings dry out and slip, crack or break. They are not expensive and readily available and I always replace them as a matter of course when renovating any Heathkit using o-rings (HW-100, HW-101, SB- series).

One problem with the Preselector o-rings is that they tend to have "backlash" due to stretch in the rubber o-ring and the tuning is not the same when turning clockwise as when turning counter-clockwise.

On some rigs, the Final Load control is operated via a dial cord. This is generally a good thing as it avoids slippage. When an o-ring is used, it places strain on both the plastic shaft coupler for the Final Tune capacitor and front bearing in the Final Load capacitor. That being said, it is a bit of a pain to string the dial cord if it needs to be replaced.

The "O-Rings" on the Preselector and Final Load are often in need of replacement but have nothing to do with the main tuning. The problem is that after a while the o-rings dry out and slip, crack or break. They are not expensive and readily available and I always replace them as a matter of course when renovating any Heathkit using o-rings (HW-100, HW-101, SB- series).

One problem with the Preselector o-rings is that they tend to have "backlash" due to stretch in the rubber o-ring and the tuning is not the same when turning clockwise as when turning counter-clockwise.

On some rigs, the Final Load control is operated via a dial cord. This is generally a good thing as it avoids slippage. When an o-ring is used, it places strain on both the plastic shaft coupler for the Final Tune capacitor and front bearing in the Final Load capacitor. That being said, it is a bit of a pain to string the dial cord if it needs to be replaced.

Bob AB1MN

Good info. I overhauled an HW-101 a few years ago for a friend, and replaced a bunch of O-Rings. I did not recall where they were. But they were in pretty bad shape.

I had the same problem with my SB-102. Then I tried W1DLA's solution to the problem and it worked! I have tried this mod on other Heathkit SB rigs,,, solved the problem. W1DLA had a great simple thought out Fix !!This can be found on ehams reviews on the Heathkit 301. 73's N0CGF

This is a copy of W1DLA's fix on main tuning slippage posted on eham.

I decided to write this review, not as a complete review, but, because I don't quite understand the griping about the tuning assembly. TenTec makes a PTO that needs regular rebuilding at up to $50 a pop and gets off easier than reviews of the SB line. Slip one or two rubber o-rings into the tuning pulley on the SB-301 and it will tune smooth and steady for years...that converts it from the unreliable pinch mechanics to an edge friction dial that works great. W1DLA

Thanks everyone...your suggestions are very much appreciated. I have never owned an SB-101 and I'm not familiar with it's idiosyncracies. I had a hunch a slipping VFO dial was not a new problem. After all, this rig has been around for a long, long time and all of it's problems are bound to have surfaced by now. All I needed was to find someone who has also had a slipping dial and knew what fixed it. So many, many thanks to those who have helped with these suggestions.

Thanks everyone...your suggestions are very much appreciated. I have never owned an SB-101 and I'm not familiar with it's idiosyncracies. I had a hunch a slipping VFO dial was not a new problem. After all, this rig has been around for a long, long time and all of it's problems are bound to have surfaced by now. All I needed was to find someone who has also had a slipping dial and knew what fixed it. So many, many thanks to those who have helped with these suggestions.

Frank

I just finished restoring a SB-102. You may or may not have the same problems I had. I found 37 resistors that had gone way high in value. The resistors used in some SB-102's seem to have this problem while those used in other SB-102's do not. It is a great rig when you get it working properly.

I once owned an SB102 and found replacement O-rings from an industrial supplier on the Internet. I just measured the o-rings and found replacements for $.35 ea. Also, the phenomenon of carbon composition resistors changing to higher value with age is well known and found in many older rigs. Replace them with metal film resistors and be especially suspicious of any carbon comp. resistor used as voltage dividers and any above 100k in general.

Last time I looked metal film resistors in the 1/2 watt flavor were rated for a maximum of 350 VDC across the resistor body.... A higher voltage presents the chance of an arc over. Some restorers who want the accuracy of metal film will bump up to the 1 watt size to gain a little extra voltage margin.

Translation: They're generally OK for tube circuits, but when in doubt a carbon composition part could be the safer bet.

22K dropping resistors are the most common culprit in Heathkit HF transcievers to rise and blow the bias/gain in tube stages... but I've found a bunch of other values in the audio/oscillator area, the sideband crystal oscillator area, and the driver area that go wacko. measure 'em all is my advice if you're having issues, and if they are off in-circuit, unsolder one end and check 'em without hindrances. anything off value, install the next higher wattage, usually 1 watt.

The small carbon comps are rated for 350volts, too. But very few, if any, get more than that across them. Drift with time they will - more drift than Kon-Tiki! 350volt metal or even carbon film are much more stable.

Copyright 2000-2016 eHam.net, LLC
eHam.net is a community web site for amateur (ham) radio operators around the world.
Contact the site with comments or questions.
WEBMASTER@EHAM.NETSite Privacy Statement